1. Field of the Invention
This invention relates generally to solar control coatings and, more particularly, to a coating that has solar control and spectral properties adaptable for use on a surface other than the inner surface of the outboard sheet (#2 surface) of a multi sheet-glazed insulating unit.
2. Description of the Currently Available Technology
Multi sheet-glazed insulating glass units (“IG units”) having two or more spaced glass sheets are becoming the industry standard for residential and commercial architecture in geographic regions having cool to cold climates, e.g., those climates characterized by seasons requiring extensive periods of operating heating furnaces. The IG unit has improved thermal insulating performance over windows having single glass sheets due to its reduced conductive and convective transfer of heat compared to a conventional window. However, until fairly recently, the use of IG units has not been popular in geographic regions having warm to hot climates, e.g., those climates characterized by seasons requiring extensive periods of operating air conditioners, because the primary functionality required of windows in such regions is solar heat load reduction, not insulating value.
During the past several decades, solar-control coated glasses have been introduced into the market. Such solar-control coated glasses achieve significant levels of solar heat load reduction by decreasing the amount of solar energy (in the visible and/or near-infrared portions of the electromagnetic spectrum) that is directly transmitted through the coated glass, often by absorbing large amounts of the incident energy and/or by reflecting large amounts of visible light. More recently, certain high-performance silver-based low emissivity (low-E) coatings have been recognized as also having a significant degree of solar-control functionality in addition to their excellent thermal insulating properties. Such silver-based solar-control/low-E coated glasses are now becoming increasingly popular not only in climates characterized by long heating seasons (for their low-E/thermal insulating performance) but also in climates characterized by long cooling seasons such as the deep South, Southeastern, and Southwestern parts of the United States due to their solar-control benefits.
Today, the glazing industry desires to have window systems that have additional functionalities over and beyond thermal insulation and/or solar-control benefits (hereinafter referred to as “thermal management functionalities”). Examples of other such desired functionalities include aesthetics, safety, and ease of cleaning. For example, architects may desire to provide a wide range of colors for an IG unit to enhance the aesthetic appearance of a building. To achieve this goal, colored or tinted glass sheets can be used as the outer sheets of the IG unit. A solar control coating can be deposited on the inner surface of the outer tinted glass sheet (#2 surface of the IG unit) to provide solar control properties. An example of one suitable solar control coating is sold by PPG Industries, Inc. under the registered trademark Solarban® 60. The Solarban® 60 coating, which includes two infrared-reflecting films of silver, is discussed in U.S. Pat. No. 5,821,001.
While this standard practice of forming a solar control coating on the inner surface of the tinted glass sheet (#2 surface of the IG Unit) does provide acceptable IG units, there are some drawbacks to coating tinted glass sheets. For example, tinted glass is generally not made as often as clear glass and, therefore, may not be as readily available for coating if an architect desires a particular color for an outer pane of an IG unit in a short period of time. Moreover, if tinted glass is stockpiled in anticipation of coating, the tinted glass can develop surface deterioration or corrosion during storage. Such corrosion may not be apparent until after the tinted glass is coated and can make the coating appear mottled, stained, or otherwise unacceptable.
One technique to address the above-discussed drawbacks is to apply the solar control coating to an inner pane of a double glazed IG unit with an outer pane of an uncoated tinted glass sheet. U.S. Pat. No. 5,059,458 discloses a double glazed IG unit having an outer tinted glass sheet and an inner clear glass sheet having a silver layer on the outer surface of the clear glass sheet. However, this approach also has some drawbacks. For example, using a conventional solar control coating on the #3 surface of an IG unit (the outer surface of the inner glass sheet of a two pane IG unit) provides different solar-control performance of the overall IG unit than having the coating on the #2 surface of the IG unit (all other factors remaining the same). Specifically, putting the solar control coating on the #3 surface rather than the #2 surface results in a relative increase in the shading coefficient and a relative increase in the solar heat gain coefficient.
For example, for a reference IG unit (defined below), a conventional Solarban® 60 coating on the #2 surface can result in a luminous transmittance of 60%, a luminous exterior reflectance of 9% to 10%, a shading coefficient (ASHRAE summer conditions) of 0.35 to 0.36, and a solar heat gain coefficient (ASHRAE summer conditions) of 0.30 to 0.31. Switching the Solarban® 60 coating to the #3 surface changes the IG unit performance to a luminous transmittance of 60%, a luminous exterior reflectance of 11%, a shading coefficient (ASHRAE summer conditions) of 0.41, and a solar heat gain coefficient (ASHRAE summer conditions) of 0.36.
The performance of the Solarban® 60 coating has been well received in the IG unit field. Therefore, it would be advantageous to provide a coating, e.g., a solar control coating, that could be utilized on a surface of an inner pane of an IG unit (e.g., the #3 surface of a two pane IG unit) that provides similar solar control and/or aesthetic characteristics as those of the Solarban® 60 coating on the #2 surface of the IG unit to address at least some of the problems discussed above.